Lights at Noon

Excerpted from January 1998 issue

Using lights while the sun shines

    Charging batteries

    How many batteries?  How big?

    How costly?

    How long do the batteries last?

    But they won't last as long as car batteries


For only the cost of a lot of solar collectors and the necessary wiring, you can read by bright electric lights when the sun is shining. Running the refrigerator at night involves a few additions to the simple solar system. For that matter, it is useful to remember that the electrical grid is not a storage device. At all times, the electricity being generated is being used somewhere. 

In the view of solar advocates, solar collectors will soon be too cheap to charge for, as they imagine plummeting prices when mass production becomes a reality. 

Let's go along with that daydream for a while. Suppose there is an arbitrarily large solar collector providing electricity to an ordinary house occupied by ordinary people who use an average of 30 kWh per day. When the sun is shining, it provides electricity being used at the time, and also charges batteries that will be used when the sun isn't shining. 

Supposing that there will be frequent three-day stretches when the sun doesn't shine, we will have batteries to provide electricity amounting to about 90 kWh. One automobile battery can supply about 60 ampere-hours at 12 volts, or about 720 watt-hours. Therefore, it will take about 125 automobile batteries in new condition to supply that much energy. 

The retail cost of 125 automobile batteries is about $6000. If they last for five years, the cost would be about $1200 per year, or about $100 per month. That is, the continuing cost of the replacement batteries would about equal the continuing cost of electricity from the power company, even if the solar collectors and associated electronics were free. 

But it's not even that nice. After an automobile battery has been used in a car for a couple of years, its capacity is only about half its capacity when new. That is, a fully charged _ but two-year old _ battery may hold only 360 watt- hours. So, if our goal is to be able to supply 90 kWh for a three-day stretch two years with fully charged two-year old batteries, we'll have to have about 250 of them. We're up to $200 per month for replacement batteries. 

But that's not the end of the problems, either. 

Automobile batteries are not appreciably better (in terms of energy per unit weight) now than they were a few decades ago, but they do last longer. The reasons for their longevity are that the charging system in the car is much better, and that the batteries actually don't have to work as hard. Unlike early relay-regulated generators, today's electronically regulated alternators hold the voltage constant at all RPMs. Moreover, the battery doesn't have to work as hard because it is easier to start modern cars than 1950 Chevrolets. Lead-acid batteries last longer when they do not have to go through deep discharge and recharging cycles. 

These conditions that make today's lead-acid batteries last longer in automobiles simply do not apply to batteries to be used for energy storage for solar collectors. Every sunny day, the batteries would be charged, and every night and every cloudy day, the batteries would be deeply discharged. 

Back to reality. The colossal cost is the solar collectors

Excerpted from January 1998 The Energy Advocate
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